Lessons from the short GRB$\,$170817A - the First Gravitational Wave Detection of a Binary Neutron Star Merger

Abstract: The first, long awaited, detection of a gravitational wave (GW) signal from the merger of a binary neutron-star (NS-NS) system was finally achieved (GW$\,$170817), and was also accompanied by an electromagnetic counterpart -- the short-duration GRB 170817A. It occurred in the nearby ($D\approx40\;$Mpc) elliptical galaxy NGC$\,$4993, and showed optical, IR and UV emission from half a day up to weeks after the event, as well as late time X-ray (at $\geq 8.9\;$days) and radio (at $\geq 16.4\;$days) emission. There was a delay of $\Delta t \approx 1.74\;$s between the GW merger chirp signal and the prompt-GRB emission onset, and an upper limit of $\theta_{\rm obs}<28^\circ$ was set on the viewing angle w.r.t the jet's symmetry axis from the GW signal. In this letter we examine some of the implications of these groundbreaking observations. The delay $\Delta t$ sets an upper limit on the prompt-GRB emission radius, $R_\gamma\lesssim 2c\Delta t/(\theta_{\rm obs}-\theta_0)^2$, for a jet with sharp edges at an angle $\theta_0<\theta_{\rm obs}$. GRB 170817A's relatively low isotropic equivalent $\gamma$-ray energy-output may suggest a viewing angle slightly outside the jet's sharp edge, $\theta_{\rm{}obs}-\theta_0\sim(0.05-0.1)(\Gamma/100)^{-1}$, but its peak $\nu{}F_\nu$ photon energy and afterglow emission suggest instead that the jet does not have sharp edges and the prompt emission was dominated by less energetic material along our line of sight, at $\theta_{\rm{}obs}\gtrsim 2\theta_0$. Finally, we consider the type of remnant that is produced by the NS-NS merger and find that a relatively long-lived ($>2\;$s) massive NS is strongly disfavored, while a hyper-massive NS of lifetime $\sim1\;$s appears to be somewhat favored over the direct formation of a black hole.